1. Field of the Invention
The present invention relates to a control method for an injection motor of an injection molding machine, and more particularly, to a control method for an injection motor, which is capable of preventing the motor from overheating.
2. Description of the Related Art
In an injection molding machine, as is generally known, an injection motor is used as a drive source for an injection unit, and processes of injection, hold, and application of back pressure for metering are executed by the injection motor. In the injection molding machine of this type, the output torque of the injection motor is small, so that a very small driving current flows through the injection motor when the back pressure is applied. In the injection and hold process, a large output torque must be produced, so that a very large driving current flows through the injection motor. Meanwhile, the injection motor is selected in consideration of various design conditions, such as economic efficiency. It is not common, therefore, to use, as the injection motor, a large-capacity motor such that its continuous rated current is never exceeded by the driving current. As a result, the driving current during the injection and hold processes may possibly exceed the continuous rated current by a large margin, so that the average value of the driving current flowing through the injection motor during one injection molding cycle may sometimes exceed the continuous rated current. In such a case, since the amount of heat produced is in proportion to the square of the driving current, the injection motor may overheat or be subject to other troubles.
Control methods of an injection motor of a type capable of determining whether or not an injection motor can overheat have been proposed by the present applicant (Japanese patent application Nos. 61-72416 and 61-155174). These methods were created on the basis of recognition that a large driving current flows during the hold process and a value of the driving current during the same process is in proportion to a preset hold pressure. According to these methods, on one hand, the square of a preset hold pressure is multiplied by a hold time to obtain a first value which is in proportion to an amount of heat produced in one injection molding cycle, and on the other hand, the square of a pressure produced when an injection motor delivers its maximum torque or when a continuous rated current flows in the motor is multiplied by one cycle time to obtain a second value determination is made as to whether a ratio of the first value to the second value is less than a predetermined value to determine whether or not the injection motor can overheat.
However, according to these proposed methods, only approximation of heat produced by the injection motor is obtainable.